1. Field of the Invention
The present invention relates generally to cranes and more specifically to counter-balanced cranes with multiple suspension cables to provide stable and precise positioning of a payload.
2. Description of Related Art
Earth-based construction typically involves a large crane with a single vertical support cable which positively controls only a single degree of freedom of a payload. Laborers control the other five degrees of freedom by holding onto the payload with guy lines. Thus, human guidance is necessary for accurate positioning of the payload.
Future space missions to the Moon or to the planets will likely demand a construction crane for unloading large, massive modules from a landing site and relocating them to designated areas for construction into an operational base. The presence of astronauts for these operations will be limited because of the remoteness of these bases. Thus, the cranes designed for the unloading and moving of these modules must be capable of highly stable, precise, automated operations.
Dagalakis et al disclose in NIST Technical Note 1267 (July 1989) a design for a crane employing six suspension cables that attach to the payload. This art has the advantage of allowing positive control of all six of the degree of freedom of the payload, but has the disadvantage of requiring control of all six cables to provide precise, automated operations.
Another disadvantage of the prior art relates specifically to mobile, construction cranes. Mobile cranes require a large mass to offset the tipping moment created by lifting heavy payloads. For example, a cursory study of typical mobile cranes reveals that the ratio of the operational distance of the crane to the distance between the center of gravity of the crane and its point of tipping is typically on the order of three or four to one. Therefore, a crane designed to lift a payload of 60,000 pounds with a distance ratio of four must weigh 240,000 pounds. On Earth, such machine weight to payload weight ratios are acceptable because machine weight does not significantly affect cost. However, for planetary missions, weight is a major factor in the costs encountered for launching and thus must be reduced. Compact stowage is another design goal for cranes used in planetary missions.